Many industrial robots have at least two different operation modes: an auto mode with normal operation speed, and a teach mode with a reduced speed. The different operation modes are typically achieved with a three phase transformer equipped with two voltage outputs having different voltage levels. A DC-bus supplying current to motors at robot axes has correspondingly a nominal voltage level for the auto mode, and a reduced voltage level for the teach mode. With the lower voltage level in the DC-bus, used in teaching the robot, the robot safety is increased by preventing the motors from moving at high speeds. The two voltage levels are predetermined by the physical construction of the transformer, and they cannot be easily changed.
The voltage level in the teach mode needs to be set to correspond to a “worst case” movement i.e. the maximum motor voltage that is required during a teach sequence. As a consequence, the teach mode voltage level is still much higher than required for the majority of teach sequence movements. In practice the auto mode voltage level can be 460 V and the teach mode voltage level 50 V. Although the maximum speed in the teach mode is significantly reduced, 50 V can still cause robot movements that people experience as fast, and that can cause serious injuries to an operator.
There remains a desire to further improve robot safety by preventing unnecessary fast robot movements.